[Yum] Download only patch

[Miguel Armas]

I just attached a download only patch to Bugzilla RFE #91 
(https://devel.linux.duke.edu/bugzilla/show_bug.cgi?id=91)
The patch is against the latest CVS version

This patch adds a "downloadonly" config directive and a "-n" command line
option. 
When this option is set, yum computes dependencies and downloads all the 
needed packages, but doesn't install them.
This option is usefull if you have to install a lot of packages (distro 
update) and you have a slow link. You can leave yum downloading all night and 
do the real update next day, when you are there to fix any package that may 
brake...

RFE #91 requested a "retrieve" command to download and NOT install packages, 
so I guess this option may be usefull

BTW, this is my first Python code, so it probably isn't the cleanest way to do 
it...

Salu2!
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Miguel Armas <kuko at maarmas.com>
Consultor de Sistemas y Comunicaciones
Ing. Tec. de Telecomunicaciones
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